nslookup times out when type is set to MX

In order to send 2FA emails from a PHP web site to myself, I set up an MX record override and created a firewall rule that allows access to the mail server for the destination domain, but when I attempt to send, sendmail writes to its log that email is accepted for delivery but never delvers. To test the DNS name resolution, I run nslookup and enter type=MX command, but it always prints: ;; communications error to 10.0.0.1#53: timed out ;; communications error to 10.0.0.1#53: timed out ;; communications error to 10.0.0.1#53: timed out ;; no servers could be reached When I ping, telnet, or curl, etc, the name resolution by that DNS server works fine. For example, it can install packages from Debian repos. From that little information about sendmail, which I was able to absorb, it sounds like all it needs to deliver to admin@example.com is an MX record for example.com to point at smtp.example.com. Correct me if I am wrong. All of these items seem to be in place: there is an MX record for example.com on my DNS server; it points at smtp.example.com that also has an A record; I am sending to admin@example.com; there is a firewall rule that allows the Linux box with sendmail to resolve names with the DNS server; there is a firewall rule that allows the Linux box with sendmail to send emails to smtp.example.com. All of this exists in my LAN and does not need any access to the outside Internet. How should I troubleshoot the sendmail delivery in this situation? **EDIT**: In the mean time I found out the root cause of the failing sendmail: it prepends whatever email address I use on the command line with the currently logged in user name (without regard to su), i.e. if my user name is myusername, then it changes me@mydomain to myusername@me@mydomain on the 1st attempt and myusername@mydomain on the 2nd attempt. Something is missing from sendmail.mc, but what is it?